1. Field of the Invention
The present invention relates in general to a fluid-filled elastic mount for damping or isolating vibrations based on flows of a non-compressible fluid contained therein. More particularly, the present invention is concerned with such a fluid-filled elastic mount which is simple in construction, and is capable of exhibiting different vibration damping or isolating characteristics based on the fluid flow through differently tuned orifices, depending upon the type of the vibrations applied thereto.
2. Discussion of the Prior Art
An elastic mount is known as a vibration damping device interposed between two members of a vibration system, for flexibly connecting these two members. As a type of this elastic mount, there is known a so-called fluid-filled elastic mount as disclosed in laid-open Publication No. 55-107142 of unexamined Japanese Patent Application. The elastic mount disclosed in this publication includes a first and a second support structure which are spaced apart from each other in a load-receiving direction and are elastically connected to each other by an elastic body interposed therebetween. The elastic mount further includes a partition member supported by the second support structure so as to extend in a direction substantially perpendicular to the load-receiving direction. A pressure-receiving chamber which is partially defined by the elastic body is formed on one side of the partition member, while a variable-volume equilibrium chamber which is partially defined by a flexible diaphragm is formed on the other side of the partition member. These pressure-receiving and equilibrium chambers are filled with a suitable non-compressible fluid, and communicate with each other by an orifice passage. A pressure of the fluid in the pressure-receiving chamber changes due to elastic deformation of the elastic body upon application of vibrations in the load-receiving direction. The fluid-filled elastic mount damps or isolates the input vibrations based on the resonance of the fluid mass flowing through the orifice passage, more effectively than an elastic mount which relies upon only the elastic property of the elastic body for damping the input vibrations. For this reason, the elastic mount of this type is widely used for motor vehicles, for example, as a mount for mounting the engine on the vehicle body.
Generally, the fluid-filled elastic mount when used as a vehicle engine mount is required to exhibit a high degree of damping capability with respect to relatively low-frequency vibrations, such as engine shake and bounce, and provide a sufficiently low dynamic spring constant with respect to relatively high-frequency vibrations, such as engine idling vibrations and booming noise. However, an improvement in the vibration damping or isolating function of the mount based on the resonance of the fluid mass flowing through the orifice passage can be provided with respect to only the vibrations whose frequencies are in the neighborhood of the frequency to which the orifice passage is tuned. Where the orifice passage is tuned so that the mount exhibits a high damping effect with respect to the low-frequency vibrations, for example, the orifice passage operates as if the passage was substantially closed when the mount receives the vibrations whose frequencies are higher than the tuned frequency of the orifice passage. As a result, the elastic mount suffers from an undesirably increased dynamic spring constant, and from a considerable deterioration of the isolating function with respect to the high-frequency vibrations.
To solve the above-described problem encountered in the known fluid-filled elastic mount, it is proposed to provide two mutually independent orifice passages, one of which is tuned to provide a sufficiently reduced dynamic spring constant to isolate the high-frequency vibrations, and the other of which is tuned to effectively damp the low-frequency vibrations.
The above-indicated one of the two differently tuned orifice passage is tuned so that the resonance frequency of the fluid mass flowing therethrough is higher than that of the fluid mass flowing through the other orifice passage, and so that the flow resistance of the fluid in the one orifice passage is smaller than that of the fluid in the other orifice passage. Therefore, upon application of a vibrational load, the fluid filling the elastic mount tends to flow between the pressure-receiving and equilibrium chambers, only through the above-indicated one orifice passage having the smaller flow resistance, i.e., the orifice passage for isolating the high-frequency vibrations. Consequently, it is rather difficult to ensure a sufficient amount of the fluid flowing through the other orifice passage so as to effectively damp the low-frequency vibrations.
In view of the above drawback of the proposed fluid-filled elastic mount, the applicant of the present application proposed another fluid-filled elastic mount as disclosed in laid-open Publication No. 60-220239 of unexamined Japanese Patent Application, which includes a control valve for selectively opening and closing the above-indicated one orifice passage tuned to the higher resonance frequency, and an actuator for operating the control valve. The control valve is operated by the actuator, so that the above-indicated one orifice passage is automatically opened and closed to permit the elastic mount to exhibit different vibration damping and isolating characteristics, based on the fluid flow through the selected one of the two orifice passages, depending upon the type of the input vibration.
In the thus constructed fluid-filled elastic mount, however, the provision of the control valve disposed in the relevant orifice passage along with the actuator disposed in the elastic mount results in increased structural complexity and a comparatively large size of the elastic mount. Accordingly, the cost of manufacture of the elastic mount is inevitably increased.